Dr. Conor Patterson:

The common travel area did not operate during the period from 1930 to 1945. There was a trade war between the Irish State and Britain in the 1930s. During the Second World War, which was called the Emergency in this jurisdiction, Northern Ireland, as part of the UK, was a participant. We had armed patrols on the Border through the 1950s and 1960s. We had a hard Border through most of the 20th century. People needed approval to travel on unapproved roads. It is a red herring. This is about freedom of movement along the North-South axis.

The other point is that the Irish Republic and the UK joined the European Union on the same day for a reason. The two economies were tied and the gravitational pull of the British economy was strong on the whole island of Ireland, so the decision was made in Dublin to join and co-join with the UK. What happens in the scenario where one state will no longer be a member of the European Union and the other State will remain? How does the common travel area function in that circumstance?